Organic electroluminescent devices (hereinafter referred to as organic EL devices) have evolved rapidly from relatively simple structures to relatively complex organic EL display panels comprised of a large number of light-emitting pixels arranged in rows and intersecting columns for display of images or of alphanumeric characters. As the development of organic EL devices proceeds from a research environment via prototype fabrication to a manufacturing stage, new methods of making such devices have to be devised in a manner which is compatible with processing steps and processing equipment typically found in clean room facilities of a modem semiconductor manufacturing plant.
In one form an organic EL display device is constructed as follows: on a light-transmissive substrate are provided a series of parallel laterally spaced light-transmissive anode electrodes. An organic EL-medium is then formed on the light-transmissive substrate and on the light-transmissive electrodes. The EL-medium typically comprises several overlying thin layers of organic materials which, in combination, are capable of emitting light. A plurality of laterally spaced cathode electrodes is disposed over the EL-medium in an oriented direction with respect to the anode electrodes. Thus, a plurality of light-emitting pixels is formed in intersecting rows and columns, and a particular pixel can be stimulated to emit light when an electrical potential is applied between a particular anode electrode and a particular intersecting cathode electrode such that the anode electrode has a positive potential with respect to the cathode electrode. In this case, holes are injected from the anode electrode into the organic EL-medium, and electrons are injected from the cathode electrode into the organic EL-medium, and light emission from the organic EL-medium disposed between those electrodes results from a recombination of holes and electrons therein.
While it has been possible to pattern a plurality of light-transmissive anode electrodes (for example, anode electrodes comprised of indium tin oxide, also referred to as ITO) with high fidelity by photolithographic and etching processes, the forming of laterally spaced vapor deposited cathode electrodes of a definition and lateral spacing comparable to that achieved in forming the anode electrodes has remained a technological challenge. In early versions of organic EL devices, a cathode electrode forming material, for example, a magnesium:silver material capable of injecting electrons into the organic EL-medium layer, was vapor deposited onto the organic EL-medium layer through apertures formed in an aperture mask, whereby the aperture mask was positioned in contact with or in close proximity to the organic EL-medium layer.
A major event in forming a plurality of laterally spaced cathode electrodes was proposed by Tang et al. in commonly-assigned U.S. Pat. No. 5,294,870. In the Tang et al. disclosure, a plurality of equidistant polymeric walls or dividers are formed from a negative-working photoresist material. These walls have substantially vertical side surfaces and a top surface which is parallel with the substrate. The walls are constructed to have a certain height dimension so that an upper edge of the walls can cast a shadow relative to a line-of-sight vapor stream of a cathode electrode material which is directed under an angle at the device to be constructed.
Nagayama et al., in U.S. Pat. No. 5,071,055, disclose an organic EL display panel in which both an organic function layer (an EL-medium layer) and an overlying cathode electrode layer are formed by vapor deposition methods of respective materials through openings extending between adjacent electrical insulation ramparts which have an overhanging portion projecting in a direction parallel to a substrate.
The walls of the Tang et al. device and the ramparts of the Nagayama et al. device are constructed on the device to function as integral vapor deposition masks and, in that capacity, these masks provide a significant improvement of the method of forming laterally delineated layers of an EL-medium and of cathode electrodes when compared to the vapor deposition method through apertures in an aperture mask referred to earlier. However, the construction of such walls or ramparts as well as requirements imposed upon either the materials used for the walls or ramparts, or the patterning steps which are needed to achieve the particular cross sectional profiles of these elements, may not be compatible with the processing methods and equipment such as found in contemporary semiconductor-like manufacturing facilities.
Accordingly, it is desirable to devise methods of making an organic EL device in which an integral vapor deposition mask is formed on a substrate from materials and by process steps which are compatible with manufacturing equipment and procedures found in most contemporary semiconductor-like manufacturing facilities. Such facilities include programmable in-line track systems for coating layers of adhesion-promoting agents and of photoresist materials, for heating or baking such layers, and for developing patterns in a patternwise exposed photoresist layer.